The biosynthetic origin of the B. braunii triterpene oils has remained enigmatic. Poulter (23) suggested that the biosynthesis of the botryococcene scaffold could arise from a mechanismsimilar to that for squalene, a key intermediate in sterol and cyclized triterpene metabolism (B). Squalene biosynthesis occurs from an initial head-to-head condensation of two farnesyl diphosphate molecules (FPP) into the stable intermediate presqualene diphosphate (PSPP), followed by a reductive rearrangement to form squalene catalyzed by a single enzyme without release of the PSPP intermediate (34). Botryococcene biosynthesis is suggested to parallel that of squalene in the first half reaction, differing only in the reductive rearrangement of PSPP to yield the methyl/ethyl branched, 1′-3 linked botryococcene product.